The gasification of carbonaceous fuels, such as coals or lignite, can be used to produce a range of saleable products, such as diesel, naphtha, hydrogen and/or urea, to name but a few. In some applications, oxygen blown gasification can be preferred to air blown gasification to achieve the desired syngas (synthetic/synthesis gas) quality, to reduce the size of the equipment and to increase the performance of the downstream processing equipment.
In Integrated Drying Gasification (IDG), hot gases exiting the fluid bed gasification vessel (gasifier) at between about 750° C. and 1,050° C. are supplied to an entrained flow dryer along with partially dried coal. The dryer cools the gas down to about 200° C. to 280° C. by drying the coal. To achieve high efficiency, the moisture content of the dried coal feed to the gasifier needs to be as low as possible, preferably within the range of about 5-10%. Achieving the level of drying required for a given moisture content coal supplied to the dryer requires a set thermal energy in the gasifier off-gases. The thermal energy in the off-gases is set by the gas flow rate, the gas composition and the gas temperature. However, the use of oxygen blown gasification alters the gas composition and reduces the flow rate of the gasifier off-gases by about half compared with air blown gasification, therefore also reducing the capacity of the integrated dryer by about half. There is a desire to use the preferred oxygen blown gasification whilst addressing the drawback of reduced drying capacity.
In IDG processes, it is known to use of one or more pressurised drying vessels per gasifier, as disclosed in International Patent Publication No. WO 93/23500. To increase the pressure of the carbonaceous fuel, in particular coal, to allow it to be fed into the entrained flow dryer, a lock hopper system or train is used. Each lock hopper train consists of an atmospheric pressure hopper, a second lock hopper which cycles between atmospheric and process pressure and a third hopper which is always at process pressure. Due to size limitations on the pressure vessels, to achieve the required coal feed rate for commercial scale plant, multiple lock hopper trains are required. Known designs of the entrained flow dryer incorporate a single lock hopper train for each dryer. Therefore, the requirement for multiple lock hopper trains leads to the requirement for multiple dryers, thus adding to the complexity and cost of the process. There is a desire to minimise or avoid such added complexity and cost.
Another problem with IDG processes is that any disruption of the feed of carbonaceous fuel, such as coal, to the entrained flow dryer can negatively affect gas production in the gasifier and/or the control of key process parameters, such as fluid bed temperature. These issues can disrupt downstream processes, such as the production of saleable products or power generation. There is a desire to minimise or avoid such disruptions.
Reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form of suggestion that the prior art forms part of the common general knowledge in Australia or any other country.
Throughout this specification and claims which follow, unless the context requires otherwise, the word “comprise”, and variations such as “comprises” and “comprising”, will be understood to imply the inclusion of a stated integer or step or group of integers or steps, but not the exclusion of any other integer or step or group of integers or steps.